1. Field of the Invention
The invention relates to the field of electro-optical distance-measurement, in particular to an to an electro-optical distance-measuring unit.
2. Description of Related Art
Such electro-optical distance-measurement is known from EP-A-1 647 838, for example. The content of said application in its entirely is incorporated for elucidating the functioning of the Fizeau method for absolute distance measurement. What is of importance for the present invention is that, in this method, outgoing and returning measurement light is modulated in a modulator. The measurement light is generated by a laser, in particular a diode laser. By variation of the frequency of said modulation, a minimum of the intensity of a detected measurement light beam is determined (or, substantially synonymously, a zero crossing of the derivative of the intensity). The length of a measurement path between the measuring device and a retroreflector is determined from the minimum frequency. It is found that an additional polarization by optical elements in the measurement light beam shifts the position of said minimum as a second-order aberration. As a result, the measurement is disturbed, and compensation measures have to be implemented. The disturbance of the polarization can be caused, for example, by additional beam deflecting mirrors in the measurement beam or the retroreflector.
WO 97/18486 discloses an electro-optical absolute distance measurement according to the Fizeau method. In that case, a laser beam is generated, and guided by a focusing optical unit and an optical isolator onto a polarizing beam splitter for linearly polarizing the laser light, and is subsequently guided onto a measurement path by an electro-optical modulator, a further electro-optical crystal for eliminating the influence of an external additional polarization, and a lambda/4 retarder and an exit optical unit. Light returning along the measurement path passes through the elements mentioned as far as the polarizing beam splitter and is guided onto a detector by the latter. An evaluation unit serves for determining the length of the measurement path on the basis of the detector signal.
EP 1 744 119 describes an apparatus for optical coherence tomography. This uses light that sweeps over an optical frequency range. The light is generated by a broadband source, for example an “edge-emitting LED (ELED)” or a superluminescent diode (SLD) or a “fiber fluorescent source (EDFFS)”. The light from the source is filtered by, for example, a Fabry-Perot filter or an acousto-optical filter, such that it sweeps over a predefined frequency range with a selected frequency line. The rest of the light from the broadband source is therefore filtered out.